1. Field of the Invention
The present invention relates to a pumping device by means of which fluids or free flowing material held in a reservoir tank can be pumped to a spray gun in a metered manner.
2. Description of the Prior Art
A pumping device of this type, which is configured as a double-diaphragm piston pump, is disclosed in EP 1712796 A1. This diaphragm piston pump consists of a piston rod with a diaphragm firmly attached to each of the free ends of the piston rod. The outer area of the particular diaphragm is clamped in a housing of the pumping device, with the effect that there is a flexible ring area between the firmly clamped area and the central area of the diaphragm locked against the piston rod, in which case the flexible ring area compensates the axial movements of the piston rod in relation to the housing.
A reciprocating piston is mounted on both free ends of the piston rod, by means of which the fluid or the free flowing material is sucked out of the reservoir tank into a pumping space enclosed by the diaphragm and the housing, and then pressed out of this pumping space in a metered manner to a spray gun. The two reciprocating pistons alternately suck in and press out the fluid or the material, which means the resulting flow of fluid or free flowing material is almost constant because one reciprocating piston is sucking in the fluid or the material whilst the other reciprocating piston is pressing it out.
In order to drive the piston rod reliably, a piston is firmly connected to the piston rod between the two diaphragms. The distance between the piston and the particular diaphragm is of equal magnitude in this case. The piston, together with a first and a second housing section, closes a pressure space into which compressed air is admitted alternately through a valve, with the effect that the piston can be moved back and forth between two dead points that are chiefly formed by the first and the second housing sections.
The pumping device disclosed in EP 1712796 A1 has proven effective in practice, because the special configuration of the diaphragm makes it possible to achieve a long service life.
However, during operation of the pumping device, there is a danger that hairline cracks or other damage may come about in the diaphragm due to the permanent loadings attributable to the pumping oscillations, and that these hairline cracks or other damage may allow the fluid or free flowing material that is to be pumped to mix with the fluid guiding the diaphragm, e.g. hydraulic oil. This means that either the hydraulic oil penetrates the pumping space through the diaphragm or that, if the damage to the diaphragm structure is considerable, the fluid or material to be pumped penetrates the space for the hydraulic fluid.
This effect gives rise to significant damage within the hydraulic oil circuit, because the mixing of the fluid or free flowing material with hydraulic oil gives rise to a substance that hardens, resulting in the blockage of the pressure lines in the hydraulic oil circuit. Cleaning the hydraulic oil circuit or the pumping space is extremely time-consuming; frequently, the pumping device is rendered completely unusable by these impurities and must be renewed. However, this gives rise to considerable downtimes which must be avoided whilst the pumping device is in operation.